Recently, multilayered laminate structures such as multilayered containers or multilayered sheets in which a barrier material, e.g., a saponified ethylene/vinyl acetate copolymer or a polyamide resin, is used as a barrier layer and a polyolefin is used as an outermost layer are used in various fields. For example, fuel tanks for motor vehicles are rapidly shifting from ones made of metals to ones of a multilayered laminate structure made of synthetic resins, from the standpoints of weight reduction, capacity increase, moldability, non-rusting properties, etc. Such synthetic-resin fuel tanks are required to have fuel oil resistance, impact resistance, long-lasting adhesion, etc.
The performances required of fuel tanks are becoming severer in recent years. The fuel tanks are being required to be free from delamination or the like in the multilayered laminate structure over long and to be free from barrier layer separation, layer abnormality, or the like and inhibit fuel ingredients from volatilizing off into the air. In the case of fuel tanks made by multilayer blow molding, the fuel tanks are required to be free from the fuel volatilization attributable to the separation or the like of the pinch-off part, which is the part pinched off by a mold and is characteristic of blow molding. The fuel tanks are further required to retain low-temperature impact resistance on at least a certain level so as to minimize damage by a collision, etc.
One of processes for producing a fuel tank made of a synthetic resin is to blow-mold a synthetic resin. One technique of the blow molding of a synthetic resin comprises closing a mold around a parison, which is a tubular molten resin, to pinch the parison with the mold and blowing air into the parison to impart a shape thereto. A feature of this molding technique resides in that hollow bodies can be easily molded. However, since a resin is pinched with a mold and molded, the resultant molded article has a resin/resin fusion-bonded part called a pinch-off part. This part is a weak point with respect to the strength of the molded article. Attempts are being made to optimize the shape of the pinch-off part in order to improve the strength.
Blow-molded synthetic resin articles of a multilayer structure including a layer having the ability to prevent the contents from permeating therethrough are being molded in order to prevent the contents of the blow-molded articles from permeating through the molded articles. For example, examples of fuel tanks having such structure include the following. A synthetic-resin fuel tank is known which employs a tank main body constituted of an inner layer made of a synthetic resin, an outer layer made of a synthetic resin, and a fuel permeation-preventive layer interposed between the inner and outer layers and which has the enhanced effect of preventing fuel permeation. This synthetic-resin fuel tank is produced as a hollow container of a multilayer structure by blow molding.
This multilayered blow-molded article also has a pinch-off part, and has a problem that the contents, or a fuel, e.g., gasoline, in the case of a fuel tank, permeate through the pinch-off part. This is because the pinch-off part includes an area where no permeation-preventive layer is present, although the area is only slight. The absence of a permeation-preventive layer is attributable to the essential structure of the pinch-off part.
For preventing the permeation of contents through a pinch-off part, it has, for example, been proposed to seal a pinch-off part including an area having no permeation-preventive layer (discontinuous-barrier region) with a leak proof bead having necessary barrier properties (patent document 1). This technique produces some degree of permeation-preventive effect. However, the proposed technique is insufficient in attaining a higher degree of permeation-preventive effect and in improving the strength of the pinch-off part.
Patent Document 1: JP-T-2003-523876
A synthetic-resin fuel tank has been known which employs a tank main body constituted by superposing an inner layer made of a synthetic resin and an outer layer made of a synthetic resin through a fuel permeation-preventive layer so as to have the enhanced effect of preventing fuel permeation. Known as such a synthetic-resin fuel tank is one obtained by forming a part attachment hole in a tank main body and fusion-bonding a synthetic-resin accessory to the outer surface of the tank main body so as to cover the part attachment hole. In this case, one is known in which the accessory has a fuel permeation-preventive layer superposed on the inner surface thereof so as to enhance the effect of preventing fuel permeation.
For example, patent document 2 discloses a synthetic-resin fuel tank produced by forming a part attachment hole of a tapered shape in a tank main body so that the hole become wider toward the outer surface of the tank main body, forming on an accessory a tapered boss part to be fitted into the part attachment hole, superposing a fuel permeation-preventive layer on the inner surface of the accessory so that the layer extends to the outer surface of the boss part, and fusion-bonding the accessory to the tank main body so that the fuel permeation-preventive layer on the outer surface of the boss part comes into close contact with the fuel permeation-preventive layer exposed in the part attachment hole.
Patent Document 2: JP-A-2001-113963
Patent document 3 discloses a resin part to be attached to an opening in a fuel tank and a process for producing the resin part. This resin part is one to be attached to an opening formed in a fuel tank made of a resin, and is characterized by comprising: a resin part main body formed from a barrier resin material which prevents fuel permeation; and a jointing member which has been formed from an adhesive resin material having adhesive properties and integrally molded with the resin part main body so as to cover part of the main body and which is to be jointed to the fuel tank so as to surround the opening of the fuel tank.
The part structure produces some degree of permeation-preventive effect. However, it is insufficient for obtaining a higher degree of permeation-preventive effect. In addition, the alcohol contained in the fuel swells the barrier resin, resulting in a decrease in barrier performance.
Patent Document 3: JP-A-2002-114047
Furthermore, patent document 4 discloses a structure of a minor-part bonding part of a synthetic-resin tank. It is a structure of that part of a synthetic-resin tank main body made of a weldable material which has an opening and at which a minor part made of a non-weldable material is bonded to the opening. The structure is characterized by forming on the minor part a flange which comes into contact with the periphery of the opening, forming from a weldable material a supporting member capable of covering the flange, and welding the supporting member to the tank main body, with the flange sandwiched between the supporting member and the tank main body.
However, in the part structure described above, the synthetic-resin tank made of a weldable material and the minor part made of a non-weldable material are not fusion-bonded to each other and sufficient sealing properties are difficult to secure. In addition, the minor part made of a non-weldable material and the supporting member are not fusion-bonded to each other and sufficient sealing properties are difficult to secure in this area also.
Patent Document 4: JP-A-2002-160538